1. Field of the Invention
The present invention relates to dielectric filters, dielectric duplexers, and communications apparatuses used mainly in the microwave band.
2. Description of the Related Art
In a known type of dielectric filter including a substantially rectangular dielectric block, the dielectric block, inner conductors, and an outer conductor constitute resonators in TEM modes, and the resonators are comb-line coupled with each other via stray capacitance generated at portions of the resonators where no conductors are formed, whereby the dielectric filter is formed.
However, in a dielectric duplexer in which an outer conductor is formed on the outer surface of such a substantially rectangular dielectric block, the dielectric block and the outer conductor cause a resonance in a mode, for example, the TE101 mode, other than the TEM mode which is the fundamental resonance mode.
FIG. 22A is a diagram showing the distribution of a magnetic field in the TE101 mode generated in the dielectric filter according to the related art, and FIG. 22B is a graph showing the attenuation characteristics of the dielectric filter.
As shown in FIG. 22B, when resonance occurs in a mode other than the fundamental mode, for example, in the TE mode, a plurality of resonance frequencies in the TE mode, in addition to the resonance frequency in the desired TEM mode, appear outside the band necessary for obtaining the desired characteristics of the filter, whereby the spurious-response characteristics of the dielectric filter are degraded.
Proposals have been made in order to avoid the effects of the TE mode. In a first proposed dielectric filter, because the frequency in the TE mode is affected by the outer dimensions of the dielectric filter, the outer dimensions are altered so as to shift the resonance frequency in the TE mode, whereby degradation of the spurious-response characteristics is avoided. In a second proposed dielectric filter, a portion of an outer conductor is cut, so that a perturbation is caused in the TE-mode resonance of the dielectric block and the outer conductor, shifting the frequency in the TE mode, whereby degradation of the spurious-response characteristics is avoided.
However, the dielectric filters according to the related art have suffered the following problems to be solved.
According to the first proposed dielectric filter, the filter must be designed for the TEM mode while also taking the effects of TE mode into consideration. In addition, because size reduction of dielectric filters is constantly desired, larger outer dimensions are inhibited. Thus, flexibility in designing filters is diminished.
In the second proposed dielectric filter, because a separate process of cutting the outer conductor is required, lead time and workload are increased, incurring additional manufacturing cost.
To address these problems, the present invention provides a dielectric filter, a dielectric duplexer, and a communications apparatus in which the resonance frequency in the TE mode is shifted so as to improve the spurious-response characteristics without incurring additional manufacturing cost or altering the overall outer dimensions.
To this end, the present invention, in one aspect thereof, provides a dielectric filter including a substantially rectangular dielectric block; a plurality of inner-conductor holes having respective apertures in a first end surface of the dielectric block and in a second end surface which is opposite to said first end surface of the dielectric block; a plurality of inner conductors formed respectively on the inner surfaces of the plurality of inner-conductor holes; at least one concavity formed either in one of the end surfaces in which the apertures of the plurality of inner-conductor holes are formed, or in one of the third and fourth end surfaces of the dielectric block which are arranged with the inner-conductor holes therebetween in the direction of array of the plurality of inner-conductor holes; and an outer conductor formed on the outer surface of the dielectric block including the inner surface of the at least one concavity; wherein the resonance frequency in a TE mode in which the electric field is aligned in the direction perpendicular to both the axial direction and the direction of array of the plurality of inner-conductor holes is shifted towards higher frequencies. Thus, the effects of TE modes can be readily diminished without altering the outer dimensions, so that the spurious-response characteristics are improved.
The at least one concavity may be formed substantially in the central portion of at least one of the first and second end surfaces in which the apertures of the plurality of inner-conductor holes are formed. Thus, mainly the effects of the TE101 mode can be readily reduced without altering the outer dimensions, so that the spurious-response characteristics are improved.
The at least one concavity may be formed in at least one of the first and second end surfaces in which the apertures of the plurality of inner-conductor holes are formed, at a position spaced away from a corresponding nearest end surface in the direction of array of the plurality of inner-conductor holes, by a distance of approximately a quarter of the dimension of the dielectric block in said direction of array of the inner-conductor holes. Thus, mainly the effects of the TE201 mode can be readily reduced without altering the outer dimensions, so that the spurious-response characteristics are improved.
The at least one concavity may be formed in a localized region not including spaces between the plurality of inner-conductor holes. Thus, the at least one concavity can be readily formed without altering the coupling capacitance between the inner-conductor holes. In addition, the effects of TE modes can be readily diminished without altering the outer dimensions, so that the spurious-response characteristics are improved.
The at least one concavity may be formed substantially in the central portion of at least one of the third and fourth end surfaces which are arranged at the ends in the direction of array of the plurality of inner-conductor holes. Thus, the effects of TE modes in general can be readily diminished without altering the outer dimensions, so that the spurious-response characteristics are improved.
The present invention, in another aspect thereof, provides a dielectric duplexer including a dielectric filter described above, so that the spurious-response characteristics can be readily improved to achieve good attenuation characteristics.
The present invention, in still another aspect thereof, provides a communications apparatus including the dielectric filter or the dielectric duplexer described above, so that the communications characteristics are improved.
Other features and advantages of the present invention will become apparent from the following description of embodiments of the invention which refers to the accompanying drawings.